Achamore was built in 1884 for Lieutenant-Colonel William James Scarlett, a Crimean War veteran whose family had owned Gigha since 1856. The architect was John Honeyman of Honeyman and Keppie, a Glasgow firm that would soon employ the young Charles Rennie Mackintosh. The house is a relatively restrained Victorian country mansion in the Scottish baronial vein - turrets where you would expect them, large windows on the principal floor, an arrangement of public rooms that catches the morning light from across Ardminish Bay. A fire damaged the building in 1896 and Honeyman returned to remodel it, with completion in 1900. The Scarletts sold up in 1919. The house passed through Major John Allen, then to Richard and Elaine Hamer in 1937, then during the Second World War to Elaine's brother the writer and politician Somerset de Chair. In 1944 de Chair sold to Horlick. The house was a means. The garden was the end.

Between 1944 and his death in 1972, Horlick - the fourth baronet, grandson of one of the brothers who founded the Horlick's company in Chicago in 1873 - built up what is now recognised as one of the most significant private rhododendron collections in Scotland. The Gulf Stream-warmed climate of Gigha is exceptional: mild, sunny by Scottish standards, with sometimes fewer days of ground frost than parts of Cornwall. Horlick exploited it. He laid out the gardens in a series of woodland walks and sheltered glades, with shelterbelts of conifer to break the Atlantic wind. He commissioned plant-hunting expeditions to bring back specimens from the temperate rainforests of Chile, the Himalayan foothills, the Australian Blue Mountains. Among the things that now grow on Gigha: southern rata trees from New Zealand with their bright red bottlebrush flowers, Montezuma pines from Central America, Puya alpestris from the Andes that produces an electric-blue inflorescence, and the Wollemi pine - Wollemia nobilis - a tree thought extinct for two million years until a small grove was discovered in a New South Wales canyon in 1994. Horlick was a generous landlord who encouraged dairy farming on the island and treated his tenants well by the standards of the era. He bequeathed the garden's plant collection to the National Trust for Scotland on his death.

After Horlick the island went through a succession of private owners with varying degrees of attention to it. By the early 2000s the population had fallen to 98, the housing was in poor condition, and the islanders were tired of waking up to news of another sale to another absentee landlord. In March 2002, with funding from the National Lottery and Highlands and Islands Enterprise, the islanders bought the entire island for £4 million through the Isle of Gigha Heritage Trust. The day the purchase completed - 15 March - is celebrated locally as Independence Day. To pay off the short-term loan, the Trust sold Achamore House (but not the gardens) to Don Dennis, a Californian businessman who now runs a flower essences business from it and lets some of the rooms as bed and breakfast. The gardens remain in the Trust's ownership and are open to the public. The Achamore Garden Trust manages day-to-day operations. Visitors arrive by ferry at Ardminish, walk the half-mile inland, and find themselves in fifty-four acres of rhododendrons, magnolias, eucryphia, and trees from four continents that should not by all rights be growing this far north in Britain.

The best time to visit Achamore is May and June, when the rhododendrons - more than 50 named cultivars in the collection, some unique to Gigha - hit their full intensity of colour. The walls of pink and crimson and lilac in the main woodland garden are the kind of thing that stops first-time visitors mid-conversation. June through September brings the eucryphias into bloom, their white flowers carrying lightly into the woodland on warm evenings. The Wollemi pine - planted as one of the first specimens distributed to British botanic gardens after the rediscovery in Australia - stands in a sheltered glade and is now several metres tall. The southern rata are slower; rata in their native New Zealand can live a thousand years, and Horlick's plantings are still effectively saplings. The gardens hold national plant collections of certain rhododendron groups and are recognised on the Inventory of Gardens and Designed Landscapes in Scotland. The Horlick fortune came from convincing Edwardian Britain that a glass of warm malted milk before bed was a sensible nightly ritual. The Horlick taste, you discover walking the gardens, was for cataloguing the temperate-zone flora of the world and seeing what would grow on a small island in the Inner Hebrides. Most things, it turns out, will.